The present invention relates to an apparatus for controlling a stepping motor used to position, for example, the magnetic head of a floppy disk drive.
A stepping motor (usually called a "pulse motor") can easily be controlled and is used in various positioning devices. It is used in a floppy disk drive to move a magnetic head to the desired track of a magnetic disk, so that the head may write data on the track or read the data therefrom. Moving the head to a desired track is generally called the "seek operation."
When the floppy disk drive is accessed by a host computer, the magnetic head starts a read/write operation at the track where it is positioned or is moved by the stepping motor to another track and then starts the same operation at this track. The torque of the motor required for the seek operation is greater than the torque the motor must apply to hold the head at any track. In the conventional method, a high excitation voltage is applied to the motor for the seek operation, and the motor is driven by a low excitation voltage to hold the head at any track. If the same excitation voltage is applied to the motor for both operations, more power than necessary will be inevitably consumed. To save power further, no power is supplied to the motor while the disk drive remains to be accessed, which condition will be called the "standby state".
To make the magnetic head write on or read from the same track after the disk drive has come out of the standby state, a low excitation voltage is applied to the stepping motor. As shown in FIG. 1A, the angle .theta., by which the rotor 13 of the motor rotates with respect to the stator 14 to generate a predetermined torque T (FIG. 1B), is greater when the low excitation voltage (e.g., 5 V) is applied to the motor as shown by curve 10 in FIG. 1A than when a high excitation voltage (e.g., 12 V) is applied to the motor as shown by curve 11, provided the motor exerts the same torque Ti. Hence, the head cannot be positioned as accurately during the low voltage application as during the high voltage application. The known method of controlling a stepping motor can save power during the seek operation since the excitation voltage is low during this operation, but it has the abovementioned drawback.
To read data from and write data on the same track after the disk drive has come out of the standby state, the low excitation voltage is applied to the stepping motor to hold the head at the track. As stated above, however, angle .theta. is greater during the low voltage application than during the high voltage application. Hence, the head may not be so accurately positioned after the disk drive has been released from the standby state.